** Railroad operator Union Pacific's UNP.N shares fall 3% to $223.99, Norfolk Southern's NSC.N rise 3.5% to $290

** UNP said it is in talks with rival NSC for a merger which would create a transcontinental railroad behemoth

** If the two companies get into a deal, it would be largest-ever buyout in the sector

** As of last close, UNP had risen 1.3% YTD and NSC rose 19.4% YTD

(Reporting by Aatreyee Dasgupta)

((Aatreyee.Dasgupta@thomsonreuters.com))